Birds
Common Poorwill
Phalaenoptilus nuttallii · also called poorwill, the sleeper
Birds do not hibernate. That was the rule until 1946, when a naturalist in the California desert found a small grey nightjar sitting cold and unresponsive in a rock crevice, and kept finding it there, winter after winter. The common poorwill is the exception, and it is still the only one.
Overview
Phalaenoptilus nuttallii is the smallest nightjar in North America, about 20 cm, grey-brown and marbled like a lichen-covered stone. It lives in the dry interior of the West — sagebrush, chaparral, rocky desert slopes — and hunts moths and beetles in flight around dusk and dawn.
The hibernating bird
It is the only bird known to enter true hibernation. In cold weather it wedges into a crevice, lets its body temperature fall to around 5 degrees Celsius, cuts its metabolism to a fraction of normal, and stays dormant for days, weeks, or occasionally months, living off autumn fat. The Hopi name for it, Holchoko, means roughly “the sleeping one.”
On the ground
It builds no nest, laying two eggs on bare gravel, and roosts motionless on the ground by day, trusting its camouflage until an intruder nearly steps on it.
Status
Least Concern and widespread across the arid West, though hard to survey and often killed on warm night roads.
Common Poorwill: common questions
Is the common poorwill really the only bird that hibernates?
It is the only bird known to hibernate in the full sense, meaning a deliberate, prolonged, deep dormancy lasting weeks rather than a single cold night. Many small birds, including hummingbirds and other nightjars, use nightly torpor to save energy, cooling down for a few hours until morning. The common poorwill takes this much further. In the colder parts of its range it can settle into a rock crevice or a shallow scrape in autumn, let its body temperature fall close to that of its surroundings, and remain there, barely breathing, for most of the winter, rousing slowly when warm weather returns. Some individuals may instead migrate, so the same species does both.
How does a hibernating poorwill survive without eating for weeks?
By almost switching off. In deep torpor the bird's metabolic rate falls to roughly a fiftieth of its active rate, its heartbeat and breathing slow dramatically, and it stops responding to disturbance. At that pace the fat reserves it built up in autumn, when insects were abundant, are enough to carry it through long periods with no food. Warming back up to flight temperature takes several hours and uses a lot of energy, so the bird does it only when conditions are likely to stay mild. This strategy suits an insect-eater in a climate where winter nights offer nothing to catch.
Where does a common poorwill spend the day?
On the ground, motionless, relying on camouflage. It roosts among rocks, at the base of shrubs, or on bare gravel, and its grey-brown, heavily marbled plumage makes it nearly invisible against stony desert soil. It sits tight, often not flushing until an intruder is a step away, then flutters up on rounded wings, shows white tail corners, and drops back into cover a short distance off. This is also why poorwills are often seen only as a pair of orange eyeshine reflections on a quiet desert road at night, where they come to catch insects and to take grit.
What is the difference between a poorwill and a whip-poor-will?
They are related nightjars with similar names taken from their calls, but they are different birds. The whip-poor-will is larger, lives in the woodlands of eastern and southwestern North America, and gives a rapid three-note whip-poor-WILL. The common poorwill is smaller, lives in the open arid country of the West, gives a simpler two-note poor-will, and is the one that hibernates. Their ranges overlap only narrowly. A nightjar heard in sagebrush or desert foothills is almost certainly a poorwill; one heard in eastern hardwood forest is a whip-poor-will or its close relative.